1. Field of the Invention
This invention relates generally to image acquisition for automated systems and more particularly to an improved autofocus method and apparatus for such systems using a sequential multicolored illuminator.
2. Description of Related Art Including Information Disclosed under 37 CFR 1.97 and 1.98
Image acquisition systems commonly employ autofocus techniques to improve their effectiveness. A number of techniques has been employed in the past and while some of these have been effective to some extent, there is a need for improved apparatus and methods for autofocusing. While autofocusing systems and methods of the type described can be used to determine a focus position for a camera, they can also be used to measure surface features.
Known systems conventionally employ top illumination to allow a camera to determine the peak contrast in a surface area during an auto focus sweep along an axis having a component perpendicular to the plane of the object, the Z-axis. The illumination may be created with light emitting diodes including colored light emitting diodes. Colored light emitting diodes are known to enhance the features of an object's surface because some features appear differently to a camera when using different colors for surface light.
One known system analyzes small patches or tiles of pixels covering the entire video field of view (FOV). The system determines the focus Z-height, that is, the height perpendicular to the object, at which the object is in focus for each tile at the point in each tile at which the contrast is maximum for that tile. The map of the maximum contrast Z heights at all locations in the field of view describes the topology of the surface. This data can be analyzed to give the shape of a cross-section anywhere on the surface, or to provide a focus height.
Another system also uses contrast information to determine the location of the maximum Z-height in a tile and find the section shape of the area of objects. This system can link to the Z-height data from several field of view data sets from a number of auto focus sweeps together to analyze the form of object surfaces larger than one field of view.
These known systems produce data with accuracy that depends on the characteristics of the surface being examined. When a tile size is small, for example 4×4 pixels, to obtain dense Z-height data from a surface, the structure of the surface can cause the data to become unreliable due in part to inherent camera noise and the surface reflectivity variation from tile to tile.